Constitutive behaviour of copper ribbons used in solar cell assembly processes

One of the driving factors for a steady reduction in wafer and cell thickness is the present shortage of polysilicon feedstock combined with the need to reduce manufacturing costs in photovoltaic module production. Therefore materials and manufacturing processes must adapt to maintain acceptable mechanical yields and module reliability. The soldering of solar cell strings is a critical step in the production of photovoltaic modules. Mechanical load and temperature induced stresses can cause cracking in the cells. During the soldering operation, the cell and the wires heat up and expand and then later contract when the heat is removed below the melting point of the solder. The differential contraction between the Cu and the Si combined with thermal gradients, cause stress to build up in the system. Since the solder thickness (5 ... 20 mu m) is relatively small compared to thickness of the copper ribbon (100 ... 200 mu m) and the thickness of the silicon solar cell (160 ... 200 mu m), the constitutive behaviour of the copper ribbons is one of the key factors to reduce breakage after soldering of solar cells into strings.